This is another look from the bacteria angle and is being applied to all corals, not just Acropora. I'm of the opinion that Acropora are more fragile than other SPS and are more easily infected by bacterial disease. I think the last sentence is interesting.
http://reefkeeping.com/issues/2002-03/eb/index.php
"Several years ago, an article appeared that described the bacteria,
Vibrio shiloi, as causing bleaching in the Mediterranean coral,
Oculina patagonica (Kushmaro 1996). At the time, most people were of the opinion that the conditions of this were unusual. It seemed to occur in a single species in a non-coral reef area. Most researchers were relatively unconcerned. Julian Sprung spoke vocally about this event in a discussion on NOAA's coral-list, and it was similarly met with some skepticism that it could be much of an issue for corals, in general. To be sure, I was one of them. "
"However, one could have heard a pin drop during the elegant and outstanding presentation by Dr. Eugene Rosenberg of Tel Aviv University (Rosenberg 2002). This man single handedly threw the proverbial monkey wrench into the coral world that morning. In the years since the original articles have been published, Rosenberg's team has not only fulfilled Koch's postulates for this pathogen in a textbook-like fashion, but has proceeded to describe the etiology in an extremely impressive manner. "
"In short, Vibrio shiloi is a newly described species of bacteria, related to V. mediterranei, with an as-yet undetermined reservoir; that is, it is not known where or if the presence of this bacteria is normal to the environment, or if it is somehow just recently showing up to affect the area. It follows the temperature cycles of the area precisely, and causes bleaching in warm months followed by recovery as the water temperature declines."
"Interestingly, it only takes 120 bacteria to cause an infection, and the bacteria can reproduce to 109 bacteria/cm3 in five days!! With water cooling below the virulence temperature, the bacteria die rapidly. "
"The reader may ask the same question that has occurred before, and was described above. So what? It's a Vibrio that is found not on coral reefs, but is specific to one coral species that we don't keep and will likely never see. The implications are certainly interesting, but what does it mean to tropical corals? Rosenberg had an answer to this, too. Knowing the skepticism that existed in the community, he has recently gone into the Indian Ocean and the Red Sea and looked at bleached Pocillopora damicornis. Is everyone ready?
A new species of bacteria, Vibrio corallyticus, was consistently found in the tissues of the bleached Pocillopora at a level that already fulfills the first of Koch's postulates. The virulence is even more amazing. At 23° C, there are no visible signs of disease. At 25° C, bleaching occurs. At 27° C, there is rapid tissue lysis. A virulence factor is being produced by this bacteria that correlates extremely well with the temperatures commonly cited as causing coral bleaching. Furthermore, Rosenberg describes the bleaching as spreading; a characteristic seen all too often by both field observers and aquarists. "
"As a final note to this incredible tale, and as if the reader has not had enough already, Rosenberg also found that Oculina in shallow water, even in high temperature and exposed to V. shiloi, rarely bleached. They found that UV radiation acted as an effective sterilizer for V. shiloi on the coral surface! "